It is well known in the art that when kaolin clay is calcined, it undergoes a series of characteristic changes, detectable by differential thermal analysis (DTA). At about 840.degree.-1200.degree. F. (450.degree.-650.degree. C.), the clay undergoes a strongly endothermic dehydration reaction resulting in the conversion to material known as metakaolin. The metakaolin state is conveniently ascertained by acid solubility testing because the alumina in the clay is virtually completely soluble in strong mineral acid. Typically, about 45% by weight of metakaolin is soluble in hydrochloric acid of 18% strength. In contrast, solubility in hydrochloric acid of the alumina component in hydrated kaolin is very limited. Furthermore, when kaolin is calcined beyond the endotherm at higher temperatures it undergoes a characteristic exothermic reaction, resulting in phase transformation manifested by markedly reduced alumina solubility.
Calcined kaolin pigments have been used for several decades in a number of industrial applications such as paper coating, paper filling, paints, plastics, etc. In these applications they impart to the finished products a number of desirable properties: brightness, opacity, hiding power, strength (in plastics), friction (in paper). Paper coating and filling applications require almost exclusively fine fully calcined kaolin pigments such as the 93% brightness ANSILEX-93.RTM. pigment manufactured by Engelhard Corporation. See, for example, U.S. Pat. No. 3,586,523, Fanselow et al, which describes the production of such pigments from ultrafine Tertiary "hard" ultrafine kaolins. Because of high brightness and light scattering properties of these fine fully calcined kaolin pigments, their primary function in paper applications is to provide opacity and brightness, often as a replacement for much costlier titanium dioxide pigments, which can also be used to enhance these functional properties.
Although these fully calcined kaolin pigments obtained by calcining ultrafine hard kaolins are less abrasive than other calcined kaolin pigments, they are relatively abrasive when compared with available noncalcined kaolin pigments. For example, the conventional so-called "low abrasion" calcined kaolin pigments such as ANSILEX 93 typically have an Einlehner abrasion value of about 20. In practical terms this translates into increased wear of bronze web forming screens (wires) on paper making machines, dulling of paper slitter knives, wear of printing plates when they come in contact with coated paper containing fine calcined pigments in the coating formulation, and, in general, wear of any surface that comes in contact with these pigments. Paper makers are becoming increasingly demanding in their need for lower abrasion.
To overcome the abrasion problem, one can calcine the kaolin pigments at temperatures less than those required to produce pigments generally referred to by those skilled in the art as "fully calcined" pigments. In this instance, calcination temperature is controlled so that the kaolin undergoes a characteristic endothermic dehydration reaction, and the original kaolinite is fully dehydroxylated. The phase that is formed is known as "metakaolin". Calcination temperature is held significantly below that at which the metakaolin collapses as would be indicated by a sharp exotherm in the differential thermal analysis (DTA). In contrast, fully calcined kaolin pigments, such as ANSILEX 93.RTM. pigment, are calcined at temperatures above this exotherm.
It is well known, however, that the brightness of a metakaolin pigment is always poorer, generally by about 2-3%, than that of fully calcined pigments derived from the same clay calciner feed. Thus, the fully calcined version gives maximum brightness, but with poor abrasion characteristics. On the other hand, the metakaolin version has lower abrasion, but brightness is poorer.
Thus, one approach to reducing abrasion was to provide a metakaolin pigment by mechanically delaminating coarse particles of kaolin, the delamination taking place by agitating a slip of the coarse clay with plastic beads followed by calcination to metakaolin. See U.S. Pat. No. 3,519,452 Morris et al. This approach did not result in ultrabright ultralow abrasion pigments, even when abrasion was measured by the Valley abrasion, an old industry standard now replaced by more demanding Einlehner and needle abrasion testing. Brightness values of 90% were not achieved.
Brightness of calcined kaolin pigments is very strongly influenced by discoloring contaminants. The two most important ones in kaolin pigment technology are iron and titanium oxides. Typically, fully calcined kaolin pigments which are produced from fine hard middle Georgia Tertiary kaolin crudes, such as those mentioned in U.S. Pat. No. 3,586,523, carry iron and titanium contamination of about 0.90-1.1% Fe.sub.2 O.sub.3 and 1.0-1.8% TiO.sub.2, respectively. While the role of colored impurities in the brightness of calcined kaolin pigments is recognized, prior to this invention those skilled in the art were not successful using this knowledge alone to produce calcined kaolin pigments with ultrahigh brightness (e.g., 93% GE brightness or above), in combination with ultralow abrasion (e.g., 10 or lower Einlehner).
Other patents disclosing full calcination of kaolins (including mechanically delaminated kaolins) to provide pigments include: U.S. Pat. No. 3,014,836, Proctor; U.S. Pat. No. 3,058,671, Billue; U.S. Pat. No. 3,343,943, Billue; U.S. Pat. No. 3,171,718, Gunn et al; U.S. Pat. No. 4,381,948, McConnell et al. and French 1,579,130 (1969).